The present invention relates to a surface shape recognition sensor used to sense a surface shape having a fine three-dimensional pattern such as a human fingerprint or animal noseprint.
Along with the progress in information-oriented society in the environment of the current society, the security technology has received a great deal of attention. For example, in the information-oriented society, a personal authentication technology for establishment of, e.g., an electronic cash system is an important key. Authentication technologies for preventing theft or illicit use of credit cards have also been extensively researched and developed (e.g., Yoshimasa Shimizu et al., xe2x80x9cA Study on the Structure of a Smart Card with the Function to Verify the Holderxe2x80x9d, Technical Report of IEICE OFS92-32, pp. 25-30 (1992-11)).
There are various kinds of authentication schemes such as fingerprint authentication and voice authentication. Especially, many fingerprint authentication techniques have been developed so far. Fingerprint authentication schemes are roughly classified into an optical reading scheme and a scheme of using the human electric characteristic and detecting the three-dimensional pattern of the skin surface of a finger and replacing it with an electrical signal.
In the optical reading scheme, fingerprint data is read mainly using reflection of light and an image sensor (CCD) and collated (e.g., Seigo Igaki et al., Japanese Patent Laid-Open No. 61-221883). A scheme of reading a pressure difference by the three-dimensional pattern of the skin surface of a finger using a piezoelectric thin film has also been developed (e.g., Masanori Sumihara et al., Japanese Patent Laid-Open No. 5-61965).
An authentication scheme of replacing a change in electric characteristic due to contact of a skin with an electrical signal distribution by detecting a resistive or capacitive change amount using a pressure sensitive sheet so as to detect a fingerprint has also been proposed (e.g., Kazuhiro Itsumi et al., Japanese Patent Laid-Open No. 7-168930).
In the above prior arts, however, the optical reading scheme is difficult to make a compact and versatile system, and its application purpose is limited. The scheme of detecting the three-dimensional pattern of the skin surface of a finger using a pressure sensitive sheet or the like is difficult to put into practical use or is unreliable because a special material is required and fabrication is difficult.
xe2x80x9cMarco Tartagnixe2x80x9d et al. have developed a capacitive fingerprint sensor using an LSI manufacturing technology (Marco Tartagni and Robert Guerrieri, A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme, 1997 IEEE International Solid-State Circuits Conference, pp. 200-201 (1997)).
In this fingerprint sensor, the three-dimensional pattern of a skin is detected using a feedback static capacitance scheme by a sensor chip in which small capacitive detection sensors are two-dimensionally arrayed.
In the capacitive detection sensor, two plates are formed on the uppermost layer of an LSI, and a passivation film is formed on the plates. In this capacitive detection sensor, a skin surface functioning as a third plate is isolated by an insulating layer formed from air, and sensing is performed using the difference in distance, thereby detecting a fingerprint. As characteristic features of a fingerprint authentication system using this structure, no special interface is necessary, and a compact system can be constructed, unlike the conventional optical scheme.
In principle, a fingerprint sensor using a capacitive detection sensor is obtained by forming a lower electrode on a semiconductor substrate and forming a passivation film on the resultant structure. A capacitance between the skin and the sensor is detected through the passivation film, thereby detecting the fine three-dimensional pattern of the skin surface of a finger.
In this sensor chip using capacitive detection sensors, however, since a skin serves as one electrode for capacitive detection, static electricity generated at the fingertip readily causes electrostatic destruction in an integrated circuit such as a sensor circuit incorporated in the sensor chip.
To prevent the above-described electrostatic destruction of an electrostatic capacitance fingerprint sensor, a surface shape recognition sensor having an electrostatic capacitive detection sensor having a sectional structure as shown in FIG. 15 has been proposed. The sensor shown in FIG. 15 will be described. The sensor has a lower electrode 1503 formed on a semiconductor substrate 1501 via an interlevel dielectric 1502, a plate-shaped deformable upper electrode 1504 which is separated from the lower electrode 1503 at a predetermined interval, and a support electrode 1505 laid out around the lower electrode 1503 to support the upper electrode 1504 while being insulated and isolated from the lower electrode 1503.
In the sensor having the above arrangement, when a finger to be subjected to fingerprint detection comes into contact with the upper electrode 1504, the pressure from the finger deflects the upper electrode 1504 toward the lower electrode 1503 to change the electrostatic capacitance formed between the lower electrode 1503 and the upper electrode 1504. This change in electrostatic capacitance is detected by a detection circuit (not shown) on the semiconductor substrate 1501 through an interconnection (not shown) connected to the lower electrode 1503. In this surface shape recognition sensor, when the upper electrode 1504 is grounded through the conductive support electrode 1505, static electricity generated at the fingertip and discharged to the upper electrode 1504 flows to ground through the support electrode 1505. For this reason, the detection circuit incorporated under the lower electrode 1503 is protected from electrostatic destruction.
The above-described deformable upper electrode must be formed with a space under it. An example of a sensor using such a hollow structure is described in a xe2x80x9cmethod of manufacturing a capacitive pressure sensor for detecting a change in pressure by a change in electrostatic capacitancexe2x80x9d by xe2x80x9cP. Rey et al.xe2x80x9d (reference 1: P. Rey, P. Charvet, M. T. Delaye, and S. Abouhassan, xe2x80x9cA High Density Capacitive Pressure Sensor Array For Fingerprint Sensor Applicationxe2x80x9d, proceedings of Transducers ""97, pp. 1453-1456 (1997)).
To form such a hollow structure, a lower electrode is formed, and then, a sacrificial film is formed on the lower electrode. An upper electrode and a deformable portion to which the upper electrode is fixed are formed on the sacrificial film. After that, the sacrificial film under the deformable portion is removed by etching from the sides of the edge portion of the deformable portion to which the upper electrode is fixed, thereby forming a space under the upper electrode. In such a fine hollow structure, however, since the height of the space under the deformable portion is as small as about 0.5 to 2 xcexcm, though the deformable portion generally has a length of about 50 xcexcm in the lateral direction, it is very difficult to completely remove the sacrificial film by etching from the lateral direction. Additionally, in the above-described surface shape recognition sensor, since a plurality of cells formed from a single lower electrode are arrayed, it is almost impossible to completely remove the sacrificial film by etching from the lateral direction.
To the contrary, when an opening portion is formed in the deformable portion formed on the sacrificial film, and the sacrificial film is removed by etching through the opening portion, the sacrificial film can be efficiently removed. Hence, the sacrificial film can be completely removed.
When the upper electrode serving as a deformable portion has an opening portion, a foreign substance or the like may enter the hollow structure from the opening portion to impede the detection operation of the sensor. This may also cause an error in the lower electrode. Hence, in such a surface shape recognition sensor, to close (seal) the opening portion, a protective film is formed on the upper electrode.
However, the opening portion poses a problem in forming a protective film for protecting the upper electrode on the upper electrode. As the protective film, an inorganic dielectric film such as a silicon oxide film or silicon nitride film is preferably used because of its characteristics. A film of such a material is generally formed by deposition such as CVD or sputtering. This method is easy to apply. However, when a protective film of an insulating material is formed on the upper electrode with an opening portion by, e.g., CVD, the insulating material enters the hollow structure from the opening portion of the upper electrode. When the insulating material enters the hollow structure to form a structure made of the insulating material, the sensor operation may be impeded by this structure in some cases.
The present invention has been made to solve the above problems, and has as its object to easily form a protective film on an upper electrode by a generally used method that is easy to apply in a surface shape recognition sensor for which a hollow structure can easily be formed by forming an opening portion in the upper electrode.
In order to achieve the above object, according to the present invention, there is provided a surface shape recognition sensor comprising a plurality of capacitive detection elements formed from lower electrodes and a deformable plate-like upper electrode made of a metal, the lower electrodes being insulated and isolated from each other and stationarily laid out on a single plane of an interlevel dielectric formed on a semiconductor substrate, and the upper electrode being laid out above the lower electrodes at a predetermined interval and having a plurality of opening portions, a support electrode laid out around the lower electrodes while being insulated and isolated from the lower electrodes, and formed to be higher than the lower electrodes to support the upper electrode, and a protective film formed on the upper electrode to close the opening portions, wherein the opening portions of the upper electrode are laid out in a region other than regions on a main part of the lower electrode and on the support electrode.